Unity Technologies has announced that from Unity 2018.1 it will no longer bundle or support MonoDevelop-Unity.

This is due to a shift to C# 6.0 which is not fully supported by this IDE.

As a consequence, starting from Unity 2018.1, Visual Studio for Mac will be included for macOS installations and Visual Studio 2017 Community will continue to be included for windows while MonoDevelop-Unity will be dropped as an alternative.

More on the Unity blog.